A conventional heat pump hot-water supply system is disclosed in Patent Document 1. In the conventional example disclosed in Patent Document 1, a heat pump hot-water supply device is operated in the nighttime during which the electricity rate is low, so that hot water to be used on the next day is stored in a storage tank.
An indicator that shows the performance (capability) of the heat pump hot-water supply device is exemplified by a coefficient of performance (COP). The COP is a ratio of an effective heating capacity to power consumption when the heat pump hot-water supply device is operated. For example, the COP of the heat pump hot-water supply device that has a heating capacity of 4.5 KW at a power consumption of 1.2 KW is calculated as 4.5÷1.2≈3.7. This COP of a given heat pump varies depending on the heat value obtainable from the outside air by the heat pump hot-water supply device, and rises as the temperature of the outside air increases. That is, if the heating capacity is kept constant, the power consumption of the heat pump hot-water supply device required to obtain the same heat value (amount of hot water) decreases in proportion to an increase in the outside air temperature.
However, since the conventional system disclosed in Patent Document 1 whose main concern is to reduce the operating cost operates the heat pump hot-water supply device in the nighttime during which the electricity rate is low, it may not be able to achieve power (energy) savings.
[Patent Document 1] Japanese Patent Application Publication No. H9-68369